Investigations of the subunit structure of chromatins will be continued along several lines: 1. Comparative studies of chromatin structure, utilizing digestion by variety of nucleases, in a variety of eukaryotic cell types. These will include yeast cell, chicken erythrocytes, HeLa cells, sea urchins, and dinoflagellates. The existence of common and divergent structural features will be sought. 2. Reconstitution of nucleosomal core particles and multisomes of defined structure will be carried out, using both unmodified and modified histones. The conditions for structural integrity of these structures will be explored. 3. Cell cycle dependent changes in chromatin and nucleosomal structure will be examined, using HeLa and yeast cells. Particular emphasis will be placed on structure present during S-phase in HeLa cells and meiosis in yeast. BIBLIOGRAPHIC REFERENCES: K.E. Van Holde and I. Isenberg "Histone Interactions and Chromatin Structure" Accounts of Chemical Research 8, 327 (1975). Barbara Ramsay Shaw, T.M. Herman, R.T. Kovacic, G.S. Beaudreau, and K.E. Van Holde, "Analysis of Subunit Organization In Chicken Erythrocyte Chromatin" Proc. Nat. Acad. Science (USA) 73, 505 (1976).